It is generally known that automatic transmissions mounted on vehicles may include a hydraulic power transmission device such as a torque converter, coupled to a driving source such as an engine, and a transmission mechanism coupled to the hydraulic power transmission device and provided with a plurality of planetary gear sets (planetary gear mechanisms) and a plurality of friction engaging elements, such as clutches and brakes. The plurality of friction engaging elements are selectively engaged by a hydraulic control to achieve a plurality of gear positions with different gear ratios.
In recent years, there is a tendency to eliminate the hydraulic power transmission device because of the desire for an increasing number of gear positions, and a reduction of the weight, etc., of the automatic transmission. In this case, it is possible to realize a smooth start of traveling by carrying out a slip control of at least one of the friction engaging elements which is engaged at a first gear when the vehicle starts traveling, while avoiding an engine stall.
When carrying out the slip control of the friction engaging element engaged at the first gear when the vehicle starts traveling, since the brake that causes the hydraulic chamber not to rotate is better in controllability at the time of the engagement than the clutch that causes the hydraulic chamber to rotate, it is possible to carry out slip control of the brake engaged at the first gear when the vehicle starts traveling.
Among the brakes constructed in this way, it is known that a piston which causes a friction plate to be engaged is biased and moved in the engaging direction by a spring in order to improve the response when the vehicle starts traveling.
For example, JP2017-150533A discloses a brake in which a piston which causes a plurality of friction plates to be engaged is biased by a first spring and a second spring in the engaging direction from the released position to a first position away from a given distance from the released position, is biased only by the second spring from the first position to a second position where the plurality of friction plates become in a zero clearance state, and is biased by the hydraulic pressure for engagement from the second position to an engaged position to cause the plurality of friction plates to be engaged.
FIG. 21 illustrates a cross-sectional view of such a brake 200 in the automatic transmission. The brake 200 includes a plurality of friction plates 203 disposed between an inner stationary member 201 coupled to a transmission case and an outer rotary member 202 coupled to a given rotary member, and a piston 206 fitted into a cylinder 205 which is formed by an outer cylindrical part 204a, a flange part 204b, and an inner cylindrical part 204c of a housing 204 which are parts of the transmission case.
The brake 200 also includes a hydraulic chamber 207 for engagement to which hydraulic fluid for engagement which biases the piston 206 in the engaging direction is supplied, and a hydraulic chamber 208 for release which is disposed at the opposite side of the hydraulic chamber 207 with the piston 206 therebetween, and to which hydraulic fluid for release which biases the piston 206 in the releasing direction is supplied.
In the hydraulic chamber 207 for engagement, the first spring 209 and the second spring 210 which bias the piston 206 in the engaging direction are disposed. The second spring 210 is disposed inside a groove portion 204d formed in the outer cylindrical part 204a of the housing 204, and the first spring 209 is disposed radially inward of the second spring 210.
When engaging the brake 200, if the hydraulic pressure for release is released from a state in which the hydraulic pressure for engagement is released from the hydraulic chamber 207 for engagement, and hydraulic pressure for release is supplied to the hydraulic chamber 208 for release to move the piston 206 to the released position where the first spring 209 and the second spring 210 are compressed, the piston 206 is biased by the first spring 209 and the second spring 210 to be moved to the first position where is the given distance away from the released position in the engaging direction.
When the piston 206 reaches the first position, the piston 206 is then biased only by the first spring 209 to be moved from the first position to the second position where the plurality of friction plates 203 become in the zero clearance state. After the piston 206 reaches the second position, when the hydraulic pressure for engagement is supplied, the piston 206 is then biased by the hydraulic pressure for engagement to be moved to the engaged position where the plurality of friction plates 203 are engaged.
On the other hand, when releasing the brake 200, if the hydraulic pressure for engagement is released and the hydraulic pressure for release is supplied, from a state in which the hydraulic pressure for release is released from the hydraulic chamber 208 for release, and the hydraulic pressure for engagement is supplied to the hydraulic chamber 207 for engagement to move the piston 206 to the engaged position, the piston 206 is biased in the releasing direction and the piston 206 is moved to the released position where the first spring 209 and the second spring 210 are compressed.
In the brake 200, since the biasing force of the second spring 210 is set larger than the biasing force of the first spring 209, the piston 206 can be moved with sufficient response by the first spring 209 and the second spring 210 from the released position to the first position, and can then be moved with sufficient accuracy by the first spring 209 from the first position to the second position.
As disclosed in JP2017-150533A, in an automatic transmission provided with a brake where a hydraulic chamber for engagement is disposed at an anti-friction plate side of the piston (or the opposite side of the piston from a plurality of friction plates) which engages the plurality of friction plates disposed between an inner fixed member coupled to a transmission case and an outer rotary member coupled to a given rotary member, since the hydraulic chamber for engagement extends in the axial directions at the anti-friction plate side of the piston, the axial dimension increases.
On the other hand, in an automatic transmission provided with a brake having a piston and a hydraulic chamber for engagement which engage the plurality of friction plates disposed between the inner fixed member and the outer rotary member, the hydraulic chamber for engagement may be disposed radially inward of the inner fixed member to reduce the axial dimension.
In this case, the piston disposed at one side of the friction plates in the axial directions has a pressing part which presses the friction plates, and the pressing part is disposed radially outward of the inner fixed member. A hydraulic chamber for the engagement forming part which forms a part of the hydraulic chamber for engagement and receives hydraulic pressure for engagement is disposed radially inward of the inner fixed member. Therefore, a coupling part which couples the pressing part to the hydraulic chamber for the engagement forming part may extend radially through the one side of the inner fixed member in the axial directions.
In an automatic transmission provided with the brake structured in this way, since it needs to be compact in the axial directions in order to be mounted in a limited space of a vehicle, a spline part of the inner fixed member with which the friction plates are spline-engaged needs to secure a given axial length so as to spline-engage with the plurality of friction plates also in the released state of the plurality of friction plates.